Range imaging is presently finding increasing use in gesture recognition applications. In range imaging, a pulsed light source illuminates an object, and a gated detector array is used to obtain an image of the object. The detector array is equipped with an electronic gate or shutter that makes the detector array responsive to light only during a narrow time window when the “gate” is open. The moment of opening the “gate” is delayed by a delay time with respect to the moment the light pulse is emitted. The emitted light pulse propagates a pre-defined distance corresponding to the delay time, reflects from an object located at that distance, and propagates back. Any light reflected from an object located before or after the pre-defined distance will be suppressed by the gated detector array. The time delay is varied to obtain 3D imagery slice-by-slice.
Another approach to range imaging consists in modulating the illuminating light at a high modulation frequency and detecting, for each pixel of a detector array, a modulation phase delay between the illuminating light and light detected by the pixel. The modulation phase delay in a pixel is proportional to a distance to the object, or more particularly, the distance to a point in the illuminated scenery imaged by the pixel. At least tens of megahertz modulation rates and 10 mW level output optical power are usually required for either type of range imaging.
The modulation speed and optical power requirements make edge-emitting laser diodes preferable light sources for range imaging. Directly modulated edge-emitting laser diode chips, generating hundreds of milliwatts of infrared light, can nowadays be mass produced at a reasonably low cost, however a reliable and efficient packaging of the laser diode chips into Watt-level light sources is still relatively expensive. Powerful laser diode chips require effective removal of heat generated during normal operation. The emitted light needs to be gathered with low optical loss, reshaped for optimal illumination of an object being imaged, and directed to the object. The edge-emitting geometry of the laser diode chips, which are usually mounted on a common flat heat sink, frequently requires a complex and costly combination of high-quality turning micromirrors to direct beams emitted by individual laser chips towards the imaged object.
To incorporate a range imaging system into a gesture recognition system, for example in a gaming and/or a mobile phone application, manufacturing costs need to be dropped considerably to make the range imaging system affordable by a mass consumer. At the same time, there is a strong market pressure to miniaturize the componentry for portable consumer devices. This necessitates miniaturization of range imaging light sources, while simultaneously dropping the manufacturing costs of these light sources.
Scifres et al. in U.S. Pat. No. 4,633,476 disclose a laser diode that can emit light perpendicular to the plane of the laser chip, allowing light from multiple lasers on a common heat sink to be combined into a single, more powerful beam. Referring to FIG. 1, a laser diode 10 includes an active layer 11 sandwiched between p- and n-layers 12 and 13, respectively. The n-layer 13 includes two sub-layers, 13A and 13B. A V-shaped groove 14 is etched into the p-layer 12 and the second n-sublayer 13B from the p-layer 12 side. P- and n-electrodes 15 and 16 contact the p- and n-layers 12 and 13, respectively. The p-electrode 15 can be made sufficiently thick to serve as a heat sink. Gaps 17 are cut into the p- and n-layers 12 and 13 and into the active layer 11, to function as laser cavity mirrors. In operation, an electric current is applied between the p- and n-electrodes 15 and 16, respectively, and generated light 18 is reflected from inside the faces of the grooves 14, exiting through cut-outs 19 in the n-electrodes 16.
Among advantages of the laser diode of Scifres et al. are low profile (height) and a possibility to combine light from multiple laser diode chips. Detrimentally, however, the light source of Scifres et al. is rather difficult to manufacture. Multiple grooves and gaps need to be etched or cut into the semiconductor chip across the active layer 11, reducing yield, potentially impacting reliability, and increasing manufacturing costs.
The prior art is lacking an edge-emitting laser diode light source suitable for a range imaging system that would be inexpensive, compact, and reliable, while allowing light from many individual laser diode chips be easily combined to form a single powerful laser beam. Accordingly, it is an object of the present invention is to overcome the shortcomings of the prior art by providing an edge-emitting semiconductor light source suitable for a range imaging system.